1. Field of the Invention
The present invention relates to a method of manufacturing a plated steel sheet with a Zn--Cr composite plating, which is preferably used for automobiles, excellent in corrosion resistance and glossiness.
2. Description of the Related Art
Various countermeasures have been taken for painting and vehicle body designs to increase the corrosion resistance of automobile bodies. A rustproof steel sheet is mainly used at present in view of reliability in rustproof performance over long periods of time.
Among rustproof steel plates, a zinc-plated steel sheet or a zinc alloy-plated steel sheet is most frequently used. However, the development of a thin corrosion-resistant steel sheet which is capable of satisfying both requirements of low cost and high corrosion resistance has been desired.
As the most promising coatings to achieve the above object, Zn--Cr composite platings disclosed in Jpn. Pat. Appln. KOKOKU Publication No. 2-51996 and Jpn. Pat. Appln. KOKAI Publication Nos. 64-55397, 64-55398, 64-79393, and 3-240994 are exemplified. In these Zn--Cr composite platings, the corrosion inhibition effect by Cr present in the coatings is typical to provide a high corrosion resistance. Such a Zn--Cr composite plating is not passivated and maintains a relative base potential. For this reason, the so-called sacrificing corrosion protecting action is valid for a long period of time, and the Zn--Cr composite plating has a high corrosion resistance in a situation where the underlying steel is exposed.
However, at a joint portion of a steel sheet, unlike a thick plating material, the Zn--Cr composite plating cannot be expected to fill the joint portion of a corrosion product. The joint portion of the steel sheet is exposed to a relatively severe, nonuniform environment in which dry and wet conditions alternate. Even if the Zn--Cr composite plating is formed on the steel sheet, the resultant steel product has a lower corrosion resistance than that of a hot-dip plated steel sheet with a thick plating film.
Although the Zn--Cr composite plating has a high potential for excellent corrosion resistance, no conventional coatings which manifest such performance have been proposed.
An oxide layer is present on the surface of a Zn--Cr composite plating according a conventional method. Glossiness is not necessarily satisfactory due to the oxide layer. Since the thickness of the oxide layer easily changes depending on manufacturing conditions, the surface oxide layer becomes thick depending on plating conditions, and the uppermost plating tends peel. In general, since a plated steel sheet is always pressed, there is a high possibility of a lead oxide remaining in a mold due to the large thickness of the oxide layer.
When the Zn--Cr composite plating is generally formed by cation electrodeposition coating in the field of automobiles, electrodeposition coating causes formation of a large number of abnormal discharge points to result in coating defects because the oxide layer on the surface of the coating of the Zn--Cr composite plating is nonuniform according to the conventional method.
As described above, a steel plate with a Zn--Cr composite plating according to the conventional method does not necessarily have satisfactory properties, and a manufacturing method which can solve these problems is demanded.